The cohesiveness of water. The hydrophobic effect.

We are all familiar with the sight of oil droplets dispersed in water forming a single large oil drop. At the molecular level there is an analogous effect: nonpolar molecules tend to cluster together in water. These associations are termed the hydrophobic effect. In a sense, “water tends to squeeze nonpolar molecules together”.

The hydrophobic effect is responsible for the folding of proteins, the binding of substrates to enzymes and many other important molecular effects in biology.

The hydrophobic effect is an equilibrium effect, meaning that it can be described in terms of free energy, DG, which in turn can be divided into two components, enthaphy, DH and entropy, TDS.

DG = DH - TDS

Attractive interactions correspond to a negative DH. T must be positive. DS is positive when the system becomes more disorganized.

Enthalpy refers to attractive interactions and entropy refers to organization. For a process to be spontaneous, DG must be negative, that is the balance between DH and TDS must be such that they operate to make DG negative.

Consider two nonpolar molecule in water, initially solvated by water molecules. Then consider the two nonpolar molecules coming together to form a complex or dimer, which is solvated by water. The situation can be shown schematically as “squares” solvated on each side with a water molecule in the initial state and a water molecule on each available side in the complex. Formation of the complex requires that two water molecules are released into the bulk water solvent, where they can bond with other water molecules. The value of DH is close to zero, since the water has little interaction, attractive or repulsive with the nonpolar molecule. The release of two water molecules increases entropy (DS) and cause the –TDS to take on a significant negative value, which makes DG negative. This means that the nonpolar molecules cluster together as a complex at equilibrium. The freedom of movement of the water molecules “drives” the equilibrium. It is important to realize that nonpolar solute molecules are driven together in water not primarily because they have a high affinity for each other but rather because water bonds strongly to itself.